In conventional technology, a standard for determining a banknote jam in a process of conveying a banknote usually adopts a fixed threshold. As shown in FIG. 4, FIG. 5 and FIG. 6, in a simplified banknote conveying system, 101 represents a banknote conveying direction, 102 is a banknote conveying passage, 103 and 104 are detection sensors arranged on the banknote conveying passage, and 105 is a banknote being conveyed. S is a distance between sensors 103 and 104, L is a length of the banknote, and D is a distance between a front end of the banknote and the sensor 104. In a method for determining a banknote jam based on a fixed threshold includes, the threshold of banknote jam is K times the length of a longest banknote in a case that the banknote blocks a sensor, where K is a constant, and in a case that the banknote does not block a sensor, the threshold of banknote jam is a distance between two sensors between which the banknote is located, or K times the distance. In a case that a banknote jam occurs on a sensor, the distance D between the front end of the banknote and the sensor 104 is greater than K times the length L of the longest banknote. In a case that a banknote jam occurs between the sensors, the distance D between the front end of the banknote and the sensor 104 is greater than the distance S between the two sensors 103 and 104, or K times the distance S, i.e., 108>K*106. Therefore, in a case that a banknote jam is determined based on a fixed threshold in a process of longitudinally conveying banknotes in a longitudinal recycling system, since the banknotes differ greatly in lengths of longer edges, uncut banknotes may be mistakenly determined as a jammed banknote if the threshold is small, which reduces reliability; and if the method is based on a large fixed threshold in view of compatibility with the longest banknote or uncut banknotes, there may be a lag in determining a banknote jam, measurement accuracy may be reduced, and it is even possible that the passage is jammed or uncut banknotes (two banknotes joined head-to-tail) are mistakenly determined as a jammed banknote, etc.
Therefore, there is a need to provide a method for determining a banknote jam which can ensure reliability and improve measurement accuracy.